1. Field of the Invention
The present invention relates to an anti-lock control method and apparatus. More particularly, the present invention relates to an anti-lock control method and apparatus by which a control period of pressurization/pressure-reduction during anti-lock control can be maintained constant.
2. Discussion of the Related Art
In a vehicle anti-lock control apparatus, a control unit is generally used to ensure security of the steering property and running stability and to shorten braking distance at the time of braking. The control unit includes a microcomputer which exerts control such that a control mode of braking hydraulic pressure is determined based on an electric signal representing a wheel velocity detected by a wheel velocity sensor, and such that a hold valve consisting of a normally opened electromagnetic valve and a decay valve consisting of a normally closed electromagnetic valve, are closed/opened to thereby increase, hold, or decrease the braking pressure.
An example of such an anti-lock control apparatus is disclosed in JP-A-2-74455 (which corresponds to U.S. Pat. No. 4,984,164). This anti-lock control apparatus seeks to provide a vehicle anti-lock control method in which optimum control can be conducted in accordance with a variety of conditions. In the JP-A-2-74455 patent document, status numbers are set in accordance with various conditions of the wheel velocity so that the wheel velocity conditions are clearly sectioned for (or assigned to) the respective statuses to thereby conduct the anti-lock control for each type of status.
The anti-lock control of the above JP-A-2-74455 patent document will now be described more specifically. In that patent document, a reference velocity Vr is set in advance such that the reference velocity decreases linearly with predetermined deceleration from a velocity (Vw-.DELTA.V), which is lower than the wheel velocity Vw by a predetermined value .DELTA.V, when the wheel velocity Vw decelerated by increasing the braking hydraulic pressure reaches a predetermined deceleration. Also, a first threshold velocity VT1, which follows a pseudo body velocity Vv with a predetermined velocity difference, and a second threshold velocity VT2, are set such that the condition Vv&gt;VT1&gt;VT2 is satisfied. The pressure reduction start point is set to the point of time which is the earlier one of (1) the point of time when the wheel velocity Vw becomes lower than the reference velocity Vr and (2) the point of time when the wheel velocity Vw becomes lower than the first threshold velocity VT1. The pressure reduction end point is set to the point of time which is the earlier one of (1) the point of time when the wheel velocity Vw reaches a low peak and (2) the point of time when the wheel velocity Vw again becomes higher than the second threshold velocity VT2 in the case where the wheel velocity Vw becomes lower than the second threshold velocity VT2 for at least one time.
In such control, since the pressure reduction can be started when the wheel velocity Vw becomes lower than the first threshold velocity VT1, it is possible to obtain a stable pressure reduction start point even in the case where the wheel velocity Vw becomes low gradually. If the wheel velocity Vw becomes low rapidly, on the other hand, since the pressure reduction is started at the point of time when the wheel velocity Vw becomes lower than the reference velocity Vr, the pressure reduction can be started immediately without any delay. Further, since the point of time when the wheel velocity Vw becomes lower than the second threshold velocity VT2 is included in the pressure reducing range, time sufficient for reducing the pressure after the friction coefficient of the road surface becomes low .mu. can be obtained even in the case where the friction coefficient is changed from high .mu. to low .mu. rapidly. In this way, wheel locking can therefore be effectively prevented.
However, in the case where increase/decrease of braking hydraulic pressure is performed by the above-mentioned anti-lock control apparatus, when a slipping wheel is accelerated to the neighborhood of a body velocity, pressurization is started from that point of time in accordance with a predetermined value. This can result in the case where the control period becomes short, as shown by the dotted line in FIG. 6, because of the change of braking efficiency or the like (i.e., the braking efficiency becomes good). In the case where this phenomenon occurs such that the control period becomes short, body vibrations are caused thereby and this state is continued or maintained. Particularly in a vibration system incorporating an engine and a driven wheel coupled with one another through a driving shaft, as shown in FIG. 7, the change of the braking torque in this state causes torsional vibrations in the direction of rotation of the driving wheel. If these vibrations coincide with the natural frequency of the vibration system, there is a problem in that a resonance phenomenon is generated whereby the vibrations do not stop.